Is it possible to pay and upgrade to more powerful version?

Yes, it is possible, you can upgrade your version in user area. Upgrade cost is calculated by deducting 70% of current version price from full price of new version and taking into account number of days remained till subscription end. So, if you have CapMonster Lite with 310 days remained and want to upgrade to CapMonster Standard, the price will be:
($57 – $37 * (~0.7)) * (310 days/ 365 days) = $26.